The present invention relates to a method and apparatus for preparing a mask of an area of interest in a picture which area of interest is to be separated from the remainder of the picture. The invention also relates to a technique for digitizing a line edge of a picture particularly useful in the novel mask-preparation method and apparatus.
The purpose of mask preparation is to "cut" an area of interest and to extract it from its background. When cutting a mask, the contour line should be as smooth as possible. In addition, only the area of interest should be in the mask, to ensure that when that area is cut and placed on a different background there is no "noise" resulting from the original background. As a result of the latter requirement, users tend to cut the mask a few pixels within the boundary, or to perform a global framing operation to the whole mask when it is finished.
At the present time, the mask of a multi-color picture is generally prepared manually in the following manner: The separation photograph of the multi-color picture, on which the area of interest is the most obvious, is placed on a light table, and that area is painted with an opaque paint. The resulting film is then photographed to obtain the mask area, which is photographed again together with each separation to obtain the final picture.
In computerized systems, various tools are available for mask preparation. One tool used in mask preparation is the polygon, or smooth mask. In drawing this type mask, the operator draws a precise contour of the masked area. However, this procedure is relatively slow since the operator has to do accurate work. Although speed of the operation increases with the experience of the operator, the contour is only as good as the accuracy of the operator.
Another such tool provides the operator with means to add a pixel to a range of CMYK (cyan, magenta, yellow, black) values that determine the masked pixels. This is a fast and accurate technique, but cannot be used in most cases since the CMYK values of the area of interest within and outside the mask area overlap. In addition, since the resultant mask has only raster information and not vector information its resolution cannot be higher than the original picture. Most of the time masks prepared by this tool need to be retouched for purposes of smoothing the boundaries and cleaning up the resulting mask.
Both of the above tools are used when the picture is displayed in maximum resolution.
It will thus be seen that a mask preparation according to the existing manual procedures is very time consuming particularly if good quality masks are to be produced.
An object of the present invention is to provide a method and apparatus for preparing masks, which method and apparatus enable masks of good quality to be prepared in substantially less time.